When a recording layer constituted by a thin film of a phase-change recording material formed on a substrate is irradiated with a laser light and locally heated, the recording layer can be changed to states with optical constants that differ depending on the difference in irradiation conditions. The optical information recording medium (also referred to hereinbelow as optical recording medium) can be used for optically recording, erasing, rewriting, and reproducing information by using laser light. The optical recording media have been widely researched, developed, and marketed.
In an optical recording medium of a phase change type, the information is recorded by the state of the phase-change material constituting the recording layer is changed, for example, between a crystalline phase and an amorphous phase by the heat generated by laser light irradiation. The reproduction of information is performed by detecting the difference in the reflectance between the crystalline phase and amorphous phase.
Among the optical recording media, a rewritable optical recording medium makes it possible to delete or rewrite information by using a phase-change recording material, in which reversible phase changes are induced, for the recording layer. In the rewritable optical recording medium, the initial state of the recording layer is typically a crystalline phase. When information is recorded, the recording layer is melted by irradiation with a high-power laser light and then rapidly cooled, thereby converting the laser-irradiated portion into an amorphous phase. Meanwhile, when information is deleted, laser light with power lower than that during the recording is radiated, the temperature of the recording layer is raised, and the recording layer is then gradually cooled to convert the laser-irradiated portion into a crystalline phase. By irradiating the recording layer with laser light subjected to power modulation with high power and lower power, it is possible to record new information, while deleting the already recorded recording, that is, to rewrite information. In the rewritable optical recording medium, the amorphous portion is a mark and the crystalline portion is a space.
In a write-once optical recording medium using a material in which reversible phase changes do not occur, rewriting of information is impossible and information can be recorded only once.
A metal layer with a high thermal conductivity is used in both the rewritable optical recording media and the write-once optical recording media with the object of efficiently cooling during recording.
The reproduction of information recorded on the optical recording medium is performed by examining the difference in reflectance between the crystalline phase and amorphous phase. More specifically, the reproduction of information is performed by detecting the intensity of reflected light from the optical recording medium as a signal when the optical recording medium is irradiated with laser light that has been set to a certain constant reproduction power.
A variety of techniques for increasing the capacity of optical recording media have been studied. For example, a method is known by which minimum sizes of mark length and space length are reduced to increase the recording density. Where such a method is used, a problem is associated with the decreased S/N ratio in the reproduction signal. In addition, thermal interference occurs, which is a phenomenon in which heat generated when a mark is recorded propagates in the space portion and affects the cooling process of the adjacent mark. A problem arising when such thermal interference occurs is that edge positions of marks change and an error ratio during reproduction increases.
Further, a problem encountered even when marks and spaces of correct length are formed is that edge positions of short marks and spaces that are detected during reproduction are reproduced with a difference from ideal values due to a frequency characteristic of the reproduction optical system determined by the size of light spot. Such a shift between the detected edge and the ideal value is typically called inter-code interference. The problem arising when the size of marks and spaces is less than the light spot is that the inter-code interference becomes significant, jitter during reproduction increases, and the error ratio also increases.
Accordingly, a method has been disclosed in which recording is performed by binary driving the laser power, changing the position of the leading end of a mark according to the mark length of the mark that will be recorded and the space length of a space immediately before this mark, and changing the position of the trailing end of the mark according to the mark length of the mark that will be recorded and the space length of a space immediately after this mark (see, for example, Patent Literature 1). Such an adjustment of a control parameter of the recording pulse selected during mark recording compensates the occurrence of thermal interference between the marks during high-density recording and the occurrence of inter-code interference caused by the frequency characteristic during reproduction.
With the method representing another approach to increasing the capacity of optical recording media, a rewritable optical recording medium is used that is provided with two information layers, and information is recorded to or reproduced from the two information layers by laser light incident from one surface of the rewritable optical recording medium. By using two information layers, it is possible to double the recording capacity of the optical recording medium.
With the optical recording medium in which information is recorded on and reproduced from two information layers by laser light incident from one surface of the optical recording medium, the recording or reproduction of information on or from the information layer (referred to hereinbelow as the first information layer) that is farther from the incidence side is performed by laser light transmitted by the information layer (referred to hereinbelow as the second information layer) that is closer to the incidence side. In other words, where the transmissivity of the second information layer is low, the energy of laser light reaching the first information layer is reduced. Therefore, the reflectance from the first information layer is substantially reduced and quality of reproduced information is degraded. Likewise, the laser power necessary to record information advantageously on the first information layer increases, and when this laser power exceeds a limit set for the recording device, advantageous recording cannot be performed, and quality of recorded information is degraded.
Therefore, it is preferred that the second information layer have as high a transmittance as possible. Further, in order to increase the number of information layers with the object of increasing capacity, for example, to realize an optical recording medium provided with three or four information layers, it is necessary to increase further the transmittance of the information layers (third information layer or fourth information layer) on the laser light incidence side. Since metal layer materials typically have a high attenuation factor, it is preferred that the thickness of the metal layer of the information layer on the laser light incidence side be small in order to impart a high transmittance to the information layer on the laser light incidence side.
However, in a recordable optical recording medium, the cooling rate of the heat generated during recording typically decreases with the decrease in thickness of the metal layer. Therefore, heat propagation to the zone outside of the laser light irradiation area increases and the boundaries between the marks and spaces are blurred, thereby degrading the reproduction signal. Accordingly, a technique has been suggested in which a recording pulse is used such that the temperature drops faster when recording is performed on the farthest information layer than when recording is performed on the information layer close to the laser light incidence side (see, for example, Patent Literature 2).
However, a problem associated with the technique described in Patent Literature 2 is that the intensification of thermal interference effect caused by the increase in recording density and the number of layers in the optical recording medium cannot be prevented and a good recording characteristic is difficult to obtain in all of the recording layers.